Generally the invention pertains to measurement apparatus and methods for determining the depth of an interior, circumferential groove formed on a tubular part. For example, such a groove is produced when a certain type of fitting is joined to such a tubular part by swaging the tubular wall outwardly into an inner circumferential recess in the fitting, wherein the depth of the swaged groove is an indication of the integrity of the tube-to-fitting union.
Swaged-on fittings, such as a Harrison fitting are commonly employed to provide a secure mechanical engagement and fluid-tight seal between the outside wall surface of a tubular hydraulic line and one or more circumferential grooves provided on the interior wall of the fitting. The fitting is slipped over an undeformed end of the tube and a swaging tool is inserted into the open tube end to press the tube wall outwardly so that the wall is enlarged into the relieved groove or grooves of the fitting. A preferred mechanical joint and fluid seal is obtained by using a particular type of internally grooved fitting, called a Harrison fitting, which has a series of inner grooves, each being of generally rectangular cross section. In such case and in other similar swaged-on fittings, the effectiveness of the swaging operation in terms of providing a reliable, strong mechanical coupling along with a fluid-tight seal, depends on the extent to which the wall of the tube is deformed into the interior grooves of the fitting. Too much deformation may cause the tube wall to be partially cut by sharp edges of the rectangular cross section groove of the fitting, while too little deformation produces a weak mechanical joint and an unreliable seal.
Quality control analysis has established a correlation between the integrity of the tube-to-fitting union and a quick objective measurement of the depth of the groove formed on the interior wall of the tube relative to the inside diameter of the tube at a predetermined axial distance from the groove (i.e., approximating the nominal inside diameter). Thus, following the swaging operation, the parts are commonly inspected by using a gauge such as an inside caliper to first measure the inside diameter of the tube at a position near the groove, and then measure the inside diameter at the swaged groove, and to reject those parts which show a difference in the measurements that is either too small or too large with respect to predetermined quality control limits.
In the use of conventional gauges for this purpose, it has been found that discrepancies result from the inability of the gauging technique to consistently and accurately measure the degree of deformation associated with the swaging operation. For example, using an inside caliper having a pair of oppositely and outwardly projecting probe points mounted on an elongate probe body that is insertable into an open end of the tube yields inconsistent measurements, because of the required angular alignment of the probe with the axis of the tube to ensure that the feeler points of the caliper are in registration with precisely diametrically opposed points in the groove so as to register its maximum depth. Also, any axial offset of the probe with respect to the centerline of the tube will result in an erroneously smaller diameter reading than the maximum and actual diameter.
For these reasons, it is an object of the invention to provide a novel gauge apparatus and method for measuring the depth of the interior, swaged groove in connection with a swaged-on fitting, relative to a reference surface located inside the tube at a predetermined distance from the groove.
Another object of the invention is to provide a gauge of the above-described nature, which has a configuration that automatically causes the probe portion of the gauge to be centered within the tube so as to avoid any inaccuracies in the measurement caused by inadvertent axial offset relative to the tube's centerline.
Another object of the invention is to provide such a gauge for quality control measurement of the degree of deformation of the radially outwardly swaged wall of the tubular part, wherein the gauge is quickly, reliably and accurately zeroed prior to measuring the depth of the swaged groove.
Still another object is to provide a gauge apparatus and method having in addition to the foregoing capabilities, a construction that is durable so as to provide long troublefree life and an operation that is relatively simple so that the operator need not be highly skilled or undergo substantial training in its use.